The next-generation optical communication requires transmission of terabit data. As the speed of optical communication increases, studies and development of an optical receiver that receives an optical signal using digital signal processing have been conducted.
FIG. 1 illustrates an example of an optical receiver that receives an optical signal using digital signal processing. In FIG. 1, an optical signal transmitted through an optical transmission link is input to an optical hybrid circuit. The optical hybrid circuit obtains the I component and Q component of the optical signal by mixing local oscillation light and the optical signal. A photo detector circuit converts the I component optical signal and the Q component optical signal obtained by the optical hybrid signal respectively into an electric signal. An A/D converter converts the signals output from the photo detector into digital signals, respectively. Then, the digital signal processor recovers transmission data from the digital signals.
The digital signal processor may provide a function to compensate for the difference between the frequency of the signal light and the frequency of the local oscillation light (that is, the offset frequency), and a function to compensate for the carrier phase. In addition, the digital signal processor may also provide a function to compensate for characteristics of the optical transmission link (for example, chromatic dispersion).
As a related art, a coherent optical receiver having a mixer, O/E converter, received data processor, modulator described below has been proposed. The mixer mixes local oscillation light and received signal light. The O/E converter performs O/E conversion for the light of the mixed signal mixed in the mixer. The received data processor performs a process to obtain received data included in the received signal light through a digital signal process for the mixed signal converted into an electric signal in the O/E converter based on the first clock. The modulator modulates the local oscillation light or the received signal light using a clock with a phase synchronized with that of the first clock used for the digital signal processing in the received data processor (for example, Japanese Laid-open Patent Publication No. 2009-49613). In addition, Japanese Laid-open Patent Publication No. 2010-41210, Japanese Laid-open Patent Publication No. 2009-21887, and Japanese Laid-open Patent Publication No. 09-252283 describe other related arts.
In the transmission of super high-speed data, the signal band of the optical signal may be extended into, for example, several hundred GHz or more per wavelength. However, in the optical receiver illustrated in FIG. 1, the band of the analog receiver circuits (such as the photo detector circuit and the A/D converter) is, for example, about several dozen GHz, and it is difficult to receive the broadband optical signal as mentioned above.
In order to solve this problem, for example, a configuration in which the signal band of an input optical signal is divided into a plurality of partial bands using an optical circuit, and a plurality of analog receiver circuits respectively receive the signal in the corresponding partial band may be considered. However, in this configuration, a plurality of local oscillators to obtain the plurality of partial bands need to be synchronized. In this case, it is preferable that the phases of the plurality of local oscillation light are synchronized with each other. However, since an optical circuit that generates a plurality of local oscillator light synchronized with each other has a complicated configuration and a large circuit size, it is difficult to be implemented in the optical receiver.